Temperature Sensor Structure

ABSTRACT

A device for measuring temperature includes first and second temperature sensors enclosed in a first material having one or more material components, a contact surface for contacting a body whose temperature is to be measured, at least part of the contact surface being parallel to a lateral direction. The first and second temperature sensors are arranged at different depths from the contact surface and the net thermal conductivity across the device from the contact surface through the first and second temperature sensors is greater than the net lateral thermal conductivity of the device through the first and second temperature sensors.

BACKGROUND OF THE INVENTION

This invention relates to a device for measuring temperature,particularly the temperature of an animal or human body.

Sensors for measuring temperature are well known and includethermistors, thermocouples and semiconductor-based electronic sensors.If correctly calibrated, such sensors can provide an indication of thetemperature of an object in the region from which the sensor takes itsinputs. For example, a thermistor placed in direct contact with anobject will give an indication of the temperature of that part of theobject with which the sensor is in contact.

Often, an object does not have a uniform temperature and its measuredtemperature varies throughout its volume. For example, the temperatureof an animal or human typically varies from its core body temperature toskin temperature. Skin temperature can vary considerably withenvironmental conditions and it is therefore the core body temperaturewhich is typically more important for medical and diagnosticapplications. However, it is not always possible or convenient tomeasure core body temperature directly by invasive means. It ispreferable to make one or more measurements of an easily accessible partof the body (such as skin temperature) and estimate core bodytemperature from those measurements.

US Patent Application No. 2007/0282218 discloses a device for measuringthe local temperature of an external surface of a body using at leasttwo temperature sensors separated by an insulating layer. Themeasurements may be used to calculate core body temperature bycorrecting for a difference between core body temperature and localtemperature. Algorithms for performing such a correction in dependenceon known thermal characteristics of the body are well known in the art(for example, see “Computation of mean body temperature from rectal andskin temperatures”, Journal Applied Physiology 31: 484-489, 1971).

An example of a conventional device 12 for measuring the temperature ofa body 11 is shown in FIG. 1. Temperature sensors 13 and 14 are arrangedat different distances from the external surface 18 of body 11 inmaterial 15, and are separated by a thermally-insulating barrier 16. Theeffect of thermally-insulating barrier 16 is to cause temperaturesensors 13 and 14 to attain different equilibrium temperatures atdifferent rates, such that a measurement of the temperature of body 11can be estimated from the heat flow across the device between the firstand second sensors.

Conventional devices measure the heat flow from the subject body intothe device and require that the temperature sensors are accuratelypositioned so as to properly capture the flow of heat across the device.The accuracy of such devices is therefore heavily dependent on theaccuracy of placement of the sensors of the device. Furthermore, thedevices are readily influenced by other sources of heat in theirenvironment.

There is therefore a need for a device for measuring temperature whoseaccuracy is less dependent on the accuracy of placement of itstemperature sensors and the proximity of other sources of heat in itsenvironment.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided adevice for measuring temperature comprising: first and secondtemperature sensors enclosed in a first material having one or morematerial components; a contact surface for contacting a body whosetemperature is to be measured, at least part of the contact surfacebeing parallel to a lateral direction; wherein the first and secondtemperature sensors are arranged at different depths from the contactsurface and the net thermal conductivity across the device from thecontact surface through the first and second temperature sensors isgreater than the net lateral thermal conductivity of the device throughthe first and second temperature sensors.

Suitably, said first material has an anisotropic thermal conductivity.Preferably the thermal conductivity of the first material has ananisotropy ratio of at least 2. Preferably the first material has amaximum thermal conductivity of at least 0.5 W/m K.

Optionally, the device further comprises a second material at leastpartially enclosing the first material and having a lower thermalconductivity in the lateral direction than the first material.Preferably the first material has a greater thermal conductivity thanthe second material in the lateral direction by a factor of at least 4.

According to a second aspect of the present invention there is provideda device for measuring temperature comprising: a first material havingone or more material components; first and second temperature sensorsembedded in the first material; a second material at least partiallyenclosing the first material and having a lower thermal conductivitythan the first material; and a contact surface for contacting a bodywhose temperature is to be measured, at least part of the contactsurface being parallel to a lateral direction; wherein the first andsecond temperature sensors are arranged at different depths from thecontact surface and the first and second materials are arranged suchthat the net thermal conductivity across the device from the contactsurface through the first and second temperature sensors is greater thanthe net lateral thermal conductivity of the device through the first andsecond temperature sensors.

The second material may completely enclose the first material.Preferably the second material is thicker over the lateral extremitiesof the first material than over the contact surface and its opposingsurface. Preferably the first material is substantially disc-shaped andthe plane of the disc is substantially parallel with the lateraldirection. Preferably the second material forms a ring-shaped annulusabout the disc-shaped first material, the plane of the ring beingsubstantially coincident with the plane of the disc.

Each depth may be a distance from the contact surface to the respectivetemperature sensor along an axis substantially perpendicular to thecontact surface. Alternatively, each depth is a thermal depth defined bythe net thermal conductance from the contact surface to the respectivetemperature sensor. The first and second temperature sensors may be atthe same distance from the contact surface along an axis substantiallyperpendicular to the contact surface.

Preferably a surface of the first material provides at least part ofsaid contact surface.

Optionally, the first material comprises at least first and secondmaterial components having different thermal conductivities, the firsttemperature sensor being embedded in the first material component andthe second temperature sensor being embedded in the second materialcomponent. Preferably at least part of the contact surface is providedby the first and second material components.

Preferably the net thermal conductivity across the device is lowest inthe lateral direction. Preferably the first material is a thermallyconductive polymer.

Optionally, the contact surface supports a thin layer having a higherthermal conductivity than the first material. Preferably, in use, asurface of the first material remote from said contact surface isexposed. Optionally, said remote surface supports a thin layer having ahigher thermal conductivity than the first material.

DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a diagram of a prior art device for measuring heat flow from abody.

FIG. 2 is schematic diagram of a temperature measuring device inaccordance with a first embodiment of the present invention.

FIG. 3 is a schematic diagram of a temperature measuring device inaccordance with a second embodiment of the present invention.

FIG. 4 is a schematic diagram of a temperature measuring device inaccordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application. Various modifications to the disclosedembodiments will be readily apparent to those skilled in the art.

The general principles defined herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the present invention. Thus, the present invention is not intended tobe limited to the embodiments shown, but is to be accorded the widestscope consistent with the principles and features disclosed herein.

The present invention provides an improved device for measuringtemperature and heat flow into or out of a subject body. The device isparticularly suitable for measuring the temperature of a human or animalbody. A measure of the heat flow from a body combined with a measure ofthe temperature at the surface of that body allows the calculation of anestimate of a temperature within the body, if one knows something of thethermal characteristics of the body.

For example, the core body temperature (Tcore) of a human or animal maybe estimated from a first temperature T1 taken at a first point (such asat the skin) and a second temperature T2 measured at a second pointrelated to the first point by a known thermal transfer function. As iswell known in the art, these parameters allow the calculation of theheat flowing out of the skin in this region and can be used to estimatecore body temperature Tcore by:

Tcore=T1+A·(T2−T1)

Parameter A is typically an empirically determined coefficient whichdepends upon the thermal characteristics of the device (the thermaltransfer function) and the body tissue. Including higher order terms canfurther improve the accuracy of this estimate. The thermalcharacteristics of the device can be straightforwardly selected bydesign and measured precisely in the laboratory.

FIG. 2 shows a device 28 in accordance with a first aspect of thepresent invention. Temperature sensors 22 and 23 are mounted on PCB 24,which may or may not extend across the diameter of material component 25in which it is embedded. PCB 24 is chosen to have a similar thermalconductivity to material 25 such that the effect of its presence on theflow of heat to the first and second temperature sensors 22, 23 isminimised. Indeed, PCB 24 may be omitted if there is some other means ofconnection to the sensors, in which case material 25 extends betweensensors 22 and 23. Material 25 is partially enclosed by material 26,which has a lower thermal conductivity than material 25.

Device 28 is configured such that material 25 provides a contact surface27 that is adapted to contact the body whose temperature is to bemeasured (such as the skin of a human). Surface 27 will be referred toherein as the contact surface, and the opposing surface of a device inaccordance with the present invention, out of which heat from the bodyflows, will be referred to as the outer surface. Surface 27 may supportan adhesive or other means for attaching the device to the surface of asubject body.

In accordance with the present invention, sensors 22 and 23 are situatedat different distances from contact surface 27 such that the sensors areat different distances from body 21 (the source of heat). Preferablysensors 22 and 23 lie on a common axis perpendicular to contact surface27. This configuration assumes that the vector describing the heatgradient close to the surface of body 21 is normal to that surface.

It is preferable that material 26 does not extend completely over thecontact or outer surfaces of the device. It is advantageous if a surfaceof material 25 forms at least part of contact surface 27 such that thematerial contacts body 21 in use, and that a surface of material 25forms at least part of outer surface 27 such that the material isexposed to the environment, allowing heat to flow through the device andout of that surface. Material 26 may enclose material 25 completely, butin this embodiment, it is preferably that material 26 is thinner overthe outer and/or contact surface, or be doped in those regions with amore conductive material (such as a metal) so as to increase itsconductivity.

One or both of the contact and outer surfaces may support a thin layer(typically less than 1 mm thick) of an additional material (not shown inthe figures). This additional material may have a high thermalconductivity (for example, greater than that of material 25 andpreferably at least 10 W/mK) so as to efficiently couple (a) the contactsurface to the body whose temperature is being measured, and/or (b) theouter surface to the ambient environment. Alternatively, if theadditional material is sufficiently thin (preferably less than 0.25 mm),it may have a low thermal conductivity (possibly lower than 1 W/mK) andact as a protective layer for the respective surface, or means forsupporting (for example) an adhesive layer.

By adjusting the extent to which insulating material 26 extends over theouter surface of the device, the rates at which the sensors reach theirequilibrium temperatures can be varied. It is envisaged that the extentto which the insulating material extends over the outer surface isselected empirically, taking into account the typical range oftemperatures expected of the body and environment.

The arrangement of material components 25 and 26 is chosen such that anaxis of greatest thermal conductivity across the device is defined. Inthe embodiment shown in FIG. 2, the axis of greatest thermalconductivity is roughly perpendicular to contact surface 27, throughmaterial 25. This is because material 26, which has a lower thermalconductivity, reduces the net thermal conductivity in the directionsparallel to contact surface 27 (i.e. laterally). The axis of greatestthermal conductivity is preferably coincident with the direction of heatflow out of body 21. In other words, the direction of the vectordescribing the flow of heat from body 21 is chosen to be coincident withthe direction of highest thermal conductivity of the device when thedevice is placed in position on the body. In contrast with conventionaldevice configurations, the arrangement of the present invention helps tominimise the leakage of heat to the sensors from the lateral extremitiesof the device and ensure that it is the heat flow from the core of thebody that is measured.

It is advantageous if the thermal conductivity of material 26 is atleast 4 times smaller than that of material 25, and preferably at least10 times smaller. Material 25 preferably has a thermal conductivity ofat least 0.5 W/mK. It is particularly advantageous if material 25 is athermoconductive polymer, such as D8102 manufactured by Cool Polymerswhich has a thermal conductivity of 3 W/mK. Material 26 is preferably athermoplastic, such as polyvinyl chloride (PVC) or polyurethane (PU).

Preferably material 25 is substantially disc-shaped having greaterextent parallel to surface 27 than normal to surface 27. For example, anappropriate diameter for a patch for the human body is approximately 15mm, with the two parts of material 25 being approximately 2.5 mm andseparated by a PCB disc also 15 mm in diameter and 1 mm thick.Preferably material 26 forms a ring-shaped annulus about material 25,and in the present example is preferably a coating approximately 1 mmthick over material 25.

A device as shown in FIG. 2 may be conveniently manufactured byover-molding the temperature sensors with a thermally conductivepolymer, and then over-molding the resulting article with a thermallyinsulating polymer predominantly in a ring laterally about the disc. Inembodiments in which the sensors are mounted on a circuit board, thedevice may be constructed with a temperature sensor on each side of aprinted circuit board. A first over-molding may then be performed usinga polymer with thermal characteristics similar to or more conductivethan those of the circuit board, and a second over-molding performedusing a substantially more insulating polymer. If polymer 25 in whichthe PCB is embedded is particularly electrically conductive, a thinelectrically-insulating layer or film may be employed between the PCBand the conductive polymer.

FIG. 3 shows a second embodiment of the present invention in whichsensors 22, 23 are mounted on a flexible printed circuit board (PCB) 34.This has two advantages: firstly, device 38 can be flexible, allowingcontact surface to better conform to the contours of the externalsurface of body 21; secondly, by arranging the PCB to bend 180 degreesback upon itself (see FIG. 3), the sensors can be positioned in material25 such that only material 25 extends between the sensors and the flowof heat past the sensors is not interrupted by the PCB.

FIG. 4 shows a third embodiment of the present invention in whichmaterials 25 and 26 of devices 28 and 38 are replaced by a singlematerial 45 having an anisotropic thermal conductivity. Material 45 maycomprise multiple material components arranged so as to provide theanisotropic thermal conductivity. Sensors 22 and 23 are arranged inmaterial 45 so as to lie substantially along the axis of greatestthermal conductivity of the material and device (indicated by the dashedlines in FIG. 4). Since it is typically desired to capture the flow ofheat in a normal direction out of body 21, the axis of greatest thermalconductivity of material 45 will generally be substantiallyperpendicular to contact surface 27 of device 48.

It is advantageous if the axis of lowest thermal conductivity ofmaterial 45 is substantially perpendicular to the axis of greatestthermal conductivity so as to minimise the leakage of heat to thesensors from the lateral portions of the device.

Suitable materials having anisotropic thermal conductivity includethermally conductive polymers having substantially aligned polymerchains and a material matrix of electrically conductive components (suchas metal fibres) aligned in a polymeric insulating base material.Preferably material 45 is selected so as to have an anisotropy ratio ofat least 2: i.e. the thermal conductivity along the axis of greatestthermal conductivity is at least twice that along a substantiallyperpendicular axis of lowest thermal conductivity. Most preferably theanisotropy ratio is at least 5. Further advantageously, the thermalconductivity in all directions perpendicular to the axis of greatestthermal conductivity is substantially the same (preferably within 20%).

Contact surface 27 need not be perfectly flat and preferably is adaptedto conform to the external surface of the body whose temperature is tobe measured. If body 21 is a human or animal body, it is advantageous ifa device in accordance with the present invention is flexible so as toallow the contact surface to maintain a good contact with body 21 duringmovements of the human or animal.

In the above embodiments, it is important that the temperature sensorsare at different depths from the contact surface so that each sensorreaches a different equilibrium temperature (as required by the aboveequation for estimating the core temperature of a body). This depth maybe the perpendicular distance from the contact surface to the subjecttemperature sensor. Alternatively, or additionally, the depth may bedefined by the “thermal depth” of the temperature sensor from thecontact surface. The thermal depth is the net thermal conductance of thedevice from the contact surface to the subject temperature sensor andvaries with both the distance of the temperature sensor from the contactsurface and the thermal conductivity of the intervening material(s).

By arranging for the thermal conductance from the contact surface to bedifferent for each temperature sensor, each temperature sensor willequilibrate at a different temperature. This can be straightforwardlyachieved if material 25 comprises a first material component in which afirst temperature sensor is enclosed and a second material component inwhich a second temperature sensor is enclosed, the two materialcomponents having different thermal conductivities. Most simply,material 25 can comprise two halves: a first half of the first materialcomponent containing the first temperature sensor and a second half ofthe second material component containing the second temperature sensor,with each material component extending between the respectivetemperature sensor and the contact surface.

The term “perpendicular distance” as used herein shall be taken to meanthe distance from the specified point to the specified surface along theline normal to that surface that passes through that point.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

1. A device for measuring temperature comprising: first and secondtemperature sensors mounted on a circuit board and enclosed in amaterial comprising a first material component and a second materialcomponent, the first and second temperature sensors being embedded inthe first material component and the second material component at leastpartially enclosing the first material component and having a lowerthermal conductivity than the first material component; and a contactsurface for contacting a body whose temperature is to be measured;wherein the first and second temperature sensors are arranged atdifferent depths from the contact surface and the net thermalconductivity across the device from the contact surface through thefirst and second temperature sensors is greater than the net thermalconductivity of the device in lateral directions parallel to the contactsurface; wherein the circuit board extends substantially across thefirst material component between the first and second temperaturesensors, the circuit board and first material component havingsubstantially the same thermal conductivity of at least 0.5 W/mK.
 2. Adevice as claimed in claim 1, wherein the first material component has agreater thermal conductivity than the second material component by afactor of at least
 4. 3. A device as claimed in claim 1, wherein thesecond material component completely encloses the first materialcomponent.
 4. A device as claimed in claim 3, wherein the secondmaterial component is thicker over the lateral extremities of the firstmaterial component than over the contact surface and its opposingsurface.
 5. A device as claimed in claim 1, wherein the first materialcomponent is substantially disc-shaped and the plane of the disc issubstantially parallel to the contact surface.
 6. A device as claimed inclaim 5, wherein the second material component forms a ring-shapedannulus about the disc-shaped first material, the plane of the ringbeing substantially coincident with the plane of the disc.
 7. A deviceas claimed in claim 1, wherein each depth is a distance from the contactsurface to the respective temperature sensor along an axis substantiallyperpendicular to the contact surface.
 8. A device as claimed in claim 1,wherein each depth is a thermal depth defined by the net thermalconductance from the contact surface to the respective temperaturesensor.
 9. A device as claimed in claim 8, wherein the first and secondtemperature sensors are at the same distance from the contact surfacealong an axis substantially perpendicular to the contact surface.
 10. Adevice as claimed in claim 1, wherein a surface of the first materialcomponent provides at least part of said contact surface.
 11. A deviceas claimed in claim 1, wherein the first material component comprises atleast first and second material parts having different thermalconductivities, the first temperature sensor being embedded in the firstmaterial part and the second temperature sensor being embedded in thesecond material part.
 12. A device as claimed in claim 11, wherein atleast part of the contact surface is provided by the first and secondmaterial parts.
 13. A device as claimed in claim 1, wherein the netthermal conductivity across the device is lowest in the lateraldirections.
 14. A device as claimed in claim 1, wherein the firstmaterial component is a thermally conductive polymer.
 15. A device asclaimed in claim 1, wherein, in use, a surface of the first materialcomponent remote from said contact surface is exposed.
 16. A device asclaimed in claim 15, wherein said remote surface supports a thin layerhaving a higher thermal conductivity than the first material.
 17. Adevice as claimed in claim 1, wherein the first material component has athermal conductivity of 3 W/mK.
 18. A device as claimed in claim 1,wherein the first material component has a greater thermal conductivitythan the second material component by a factor of at least 10.